The exhaust gas generated in the operation of an electric steelmaking furnace contains a large quantity of dust and recoverable thermal energy, or heat. This exhaust gas can be used to preheat scrap that is to be charged into the electric furnace, saving both time and electric power required for melting the steel. However, oil, paint, rubber or plastic are often attached to or mixed in with the steel scrap, and heating them generates malodorous gases and fumes. Typically, the temperature of the exhaust gas drops by about 100.degree. C. to 300.degree. C. as it provides heat to the scrap being preheated. Hence exhaust gas temperature reaching the scrap has to be in the range of 650.degree. C. to 700.degree. C. which is necessary to decompose the above-mentioned organic matter in the scrap. Failure to so decompose such pollutants results in the release of malodorous effluents into the atmosphere, causing unacceptable environmental pollution.
A scrap preheating system, like the one shown in FIG. 9, has been used to remove odors from furnace exhaust gas after preheating of scrap in order to prevent environmental air pollution. Such a system typically comprises a combustion apparatus 3, wherein combustibles such as carbon monoxide, which are present in the dusty exhaust gas from furnace 2, are burned. This eliminates such constituents from the exhaust gas and heats it. Thereafter, some of this exhaust gas is introduced at a high temperature to a scrap preheater, such as 71 or 72, so that some of the heat energy from the exhaust gas may be utilized to preheat the scrap therein. An induced draft fan 73 then returns the somewhat cooled gas, after it has heated the scrap, through circulation duct 74 to a gas feeding box 75 provided upstream of the combustion apparatus 3 so that this gas may be mixed therein with dusty, high-temperature exhaust gas from the electric furnace 2. Malodorous substances picked up from the preheated scrap are thereafter burned in combustion apparatus 3 together with the combustibles, thus obtaining deordorization of the finally released exhaust gas.
As indicated in FIG. 8, the temperature of the exhaust gas from the electric furnace, however, depends on the relatively low temperature that prevails during the initial melting of the scrap after charging up of the furnace, a temperature drop by air mixture under followup-charging, a higher temperature level during the intermediate melting thereafter, a temperature drop under refollowup-charging, and an even higher temperature during the final melt thereafter. The temperature of exhaust gas going through the furnace varies in the range of 400.degree. C. to 1200.degree. C. As a result, for the period during which the temperature of the exhaust gas is at the lower end of this range, the temperature in the combustion apparatus cannot be kept at 650.degree. C. or higher, i.e., hot enough to deodorize the gas. In addition, since the gas in the combustion apparatus is reintroduced into the scrap preheater, much of the exhaust gas recirculates between the combustion apparatus and the scrap preheater, and this significantly decreases utilization of the available heat in the exhaust gas. In other words, since all the gas after preheating scrap is recirculated into the combustion apparatus in order to deodorize it, the heat energy of the exhaust gas as it comes from the electric furnace is not optimally used for preheating scrap. This eventually results in the release of malodorous effluents and lowers the overall efficiency in operating the system.
It was long believed that the gas after preheating scrap could not be deodorized without the offensive constitutents actually being burned. However, it has been confirmed by research leading to the present invention that the unpleasant odor generated by electric steelmaking furnaces is caused not only by ordinarily encountered malodorous substances other than oil but also by oil mist generated from cutting or lubricating oils that are present and mixed in with cutting scrap, etc., and that such materials have interaction effects that aggravate the malodorousness of each separate pollutant. From this viewpoint, in order to deodorize the gas after preheating scrap, it is necessary not only to remove the ordinarily encountered malodorous substances but also to separate out and collect the oil mist before released into the atmosphere. It is the purpose o the present invention to effect this in a practical system.